1. Field of the Invention
This invention relates to improved carbon fiber reinforced metal matrix composites. More particularly, the invention relates to reinforced metal matrix composites having two barrier layers on the carbon reinforcing fiber.
2. Description of the Prior Art
Stiff, strong, and thin metal and metal-like materials are needed for many engineering applications such as "skins" for aircraft, rockets and ground vehicles. Composites, consisting of a matrix of metal, alloy or intermetallic material strengthened by the inclusion of reinforcing fibers of a ceramic, metal, carbon or other material, have been found useful for these applications. Composites are often stronger and lighter than the matrix material forming the composite.
The term metal is used both specifically to refer to products made of the pure metal elements and generically to refer to products made from metal alloys of two or more elements and intermetallic compounds. As is recognized in the art, an alloy is a solid solution where the components can be present in any possible ratio within broad limits. An intermetallic is a compound in which the constituents join together in specific ranges of ratios.
Often problems occur when a reinforcing fiber is added to a matrix. For example, the reinforcing fiber can interact with the matrix during formation of or service in the composite, the fiber can release materials harmful to the matrix, or the fiber may simply dissolve in the matrix. In addition, the resulting reaction zone and the degraded fibers can make the composite brittle if there is too strong a bond between the fiber and matrix. In this application, the term reinforcing fiber is intended to refer to all strengthening inclusions such as ribbons, whiskers, fibers, platelets and the like.
Titanium metal matrix composites containing carbon reinforcing fibers are prone to these and other problems. Titanium aluminide metal matrix composites are particularly prone to these problems. Here, the term titanium metal matrix will be understood to include the pure metal, alloys, and intermetallic compounds of titanium. Barrier layers or coatings on the reinforcing fiber have been suggested as the solution to the problems of adding reinforcing fibers, particularly carbon fibers, to a metal matrix.
Katzman, in U.S. Pat. No. 4,737,382, suggests a carbide coating prepared by depositing an organometallic compound followed by pyrolysis. Ishikawa et al., in U.S. Pat. No. 4,440,571, teaches a surface treatment for inorganic fibers which comprises coating the surface with a titanate, borate, tetralkylammonium hydroxide and dextrin followed by high temperature heat treatment. As early as 1970, Sara, in U.S. Pat. No. 3,535,093, suggested silver coating carbon fibers for use in an aluminum matrix.
Hack et al., in Advanced Fibers and Composites for Elevated Temperatures, pp. 42 to 54, edited by I Ahmad, Conference Proceedings, Metallurgical Society of AIME, (1980), suggests Ti/B coatings for Al.sub.2 O.sub.3 fiber used to reinforce aluminum composites. Shindo in "Chemical Property of Carbon Fiber Surface and Interfacial Compatibility of Composites", Composite Interfaces, pp. 93-100, H. Ishida and J. L. Koenig editors, Elsevier Science Publishing CO. Inc., (1986), reports the use of dual layers of carbon and SiC to protect carbon fibers in an aluminum matrix. Shindo suggests using the carbon layer to protect the fiber from the SiC.
With regard to titanium matrix composites, commercially available SCS-6 SiC fibers made by Textron Inc. (formerly Avco) have surface layers coated with modified compositions of carbon and silicon. The purpose of the layers is to increase the fiber's compatibility with titanium alloys. Also, boron carbide coated SiC fibers have been incorporated into titanium alloys.
These suggested coatings or layers and others have not solved all of the problems of incorporating carbon fiber into a metal matrix composite, particularly if the metal is titanium. Generally, as a matrix material, titanium is a more reactive metal than aluminum. Titanium destroys most coatings. Intermetallics of titanium, such as titanium aluminide, have additional problems. When used as matrices, titanium intermetallics produce large stresses in the composite because of thermal expansion mismatch and because of their low ductility which makes them prone to matrix cracking.